Broadband microwave transmission control tube



Dec. 23, 1958 w. R. GHEN 2,356,130

BROADBAND MICROWAVE TRANSMISSION CONTROL TUBE} Filed Feb. 10, 1958 2 Sheets-Sheet 1 1 INVENTOR.

BY iwd /f" W. R. GHEN Dec. 23, 1958 BROADBAND MICROWAVE TRANSMISSION CONTROL TUBE Filed Feb. 10, 1958 2 Sheets-Sheet 2 Aft WITHOUT DIELECI'\Z\C 20D OOTM OONM

0 OPN OOUN FIZEOUEZMCV (MEGACYCLES) INVENTOR.

WlLL\AM Z. GHEN %hv Q v ATTORNEY United States Patent BROADBAND MICROWAVE TRANSMISSION CONTROL TUBE Application February 10, 1958, Serial NO. 714,093

Claims. (Cl. 315-39) The present invention relates to transmission control devices and more particularly to such devices employed in single antenna radar systems to control the propagation of microwave energy.

A device referred to as an anti-transmit-receive (ATR) tube is commonly employed to effectively direct low level returning signal energy to the receiver with a minimum of loss by absorption into the transmitter. Such ATR tubes generally comprise a section of rectangular waveguide having a length of approximately one quarter of a wavelength at the desired resonant frequency. An energy permeable dielectric member encloses one end while the opposite end is terminated by a metallic plate to define a vacuum tight envelope within which an ionizable atmosphere may be contained. An intermediate metallic member is permanently secured to the waveguide walls and extends parallel to the dielectric member. Since the position of the intermediate member is fixed, the utility of such devices has been limited to a narrow frequency range. It has, therefore, been necessary to interchange a plurality of such devices in radar systems having a tunable transmitter to operate over a wide frequency band.

The present invention has for its primary object the provision of a broadband transmission control device capable of performing over a wide frequency range.

Another object is the provision of an ATR tube which is tunable over a wide frequency range.

A still further object is the provision of an ATR tube having a tuning structure which is readily controllable, highly etficient, compact and useful in radar systems covering a wide frequency band.

The aforementioned objects as well as the features and advantages of the invention will be evident after consideration of the following description and appended drawings, in which:

Fig. 1 is a'perspective view of an illustrative embodiment with a portion of the envelope broken away;

Fig. 2 is a detailed cross section view;

Fig. 3 is a graphical presentation of results obtained in the ATR embodiments with and without the invention; and

Fig. 4 is a sectional view of an alternative embodiment of a component of the invention.

Briefly the invention discloses a control device having a rectangular waveguide envelope enclosed at one end by a gas-filled encapsulated resonant window assembly. A movable short-circuiting wall member is positioned parallel to the window assembly and defines therewith a resonant cavity. A rod of a dielectric material supports the movable wall member and extends through the envelope to a point adjacent to the window member. As the rod is rotated its positioning remains fixed while the wallmember is moved axially to vary the cavity dimens'ions and thereby tune the device to the desired resonant frequency. v

The feature of the invention resides in the fact that as the wall member moves away from the window assembly more of the dielectric material will be exposed within the resonant cavity. This results in a shortening of the guide wavelength compared to prior art structures. The increase in the average dielectric constant factor, therefore, reduces the overall amount of plunger travel required to tune the embodiment over a relatively wide frequency range. Consequently, the tuning rate will be more nearly linear.

A further advantage will be evident in that the rotating rod remains fixed at all timeswhile the wall member is adjusted. Hence, no external tuning structures need extend beyond the tube envelope as the resonant frequency is varied. Prior art metallic tuning arrangements would entail rather cumbersome lengths of rod extensions beyond the envelope. In the practice of the invention a novel structure has evolved having considerable utility in the art in many varied embodiments of transmission control tubes other than the specific ATR tube now to be described:

Referring to Figs. 1 and 2 the illustrative embodiment comprises a metallic envelope defined by a rectangular waveguide section 1 having an end wall closure 2. Flange 3 is secured adjacent to the open end of wavegulde 1 to provide means for securing a resonant window assembly shown generally at 4. The window assembly is readily demountable and is provided with an integral gas-fill. The assembly is hermetically sealed and for simplicity has been illustrated as a pair of dielectric window members 5 and 6 supported by metallic frames 7 and 8 joined together to form an envelope with a gas chamber 9. Flange 20 secured to the metallic members mates with the flange 3 to thereby support the window assembly. To assure good electrical contact with associated waveguide structure, a gasket of a woven wire mesh may abut flange 20 in the manner known in the art.

Various other embodiments of encapsulated resonant window assemblies may be incorporated, such as the dielectric'trough-shaped window shown in United States Patent No. 2,791,720 issued May 7, 1957. After evacuation desirable gaseous atmospheres such as argon and hydrogen may be employed at 'a reduced pressure of about ten millimeters.

'Another form of demountable resonant window assembly readily adaptable to the present invention is the so-called folded cylinder structure providing a thin sheath of an ionized atmosphere. This structure is disclosed in pending application Serial No. 663,536, filed June 4, 1957, and assigned to the'assignee of the present invention.

In the practice of the invention a rod 10 of a dielectric material extends within the waveguide 1 and is supported by means of a thrust bearing 11 secured to end wall 2. Bearing 11 engages the flange 12 of rod 10 to thereby permit rotation of rod 10 without any vertical displacement thereof. A slot 13 to receive a hand tool for rotation of the rod has been shown, however, a knob 01' any other suitable hand or motor operated means could be affixed to the outer end of rod 10. The rod is threaded over a major portion of its length as at 14 and is terminated at its inner end in close proximity to the,

inner dielectric window 5 surface. While a small space between the tip of rod 10 and the window has been shown this is only necessary for mechanical considerations to prevent any changes in temperature from possibly setting up some strains in the window should the rod be touching. However, contact of the rod on the window as- Patented Dec. 23, less tacting the inner wall surfaces to form a short-circuiting wall. Efficient electrical performance may also be attained by means of metallic fingers 16 secured along the peripheral edges of member 15. An insert 17 of a dielectric material is provided in member 15 to prevent any metallizing of the rod threads by successive movement of the wall member which may alter electrical performance. By selection of materials the metallizing problem may be overcome and insert 17 could be omitted.

Member 15 defines with the window assembly and contacting wall surfaces a resonant cavity 19. Upon rotation of rod 10 the member 15 will be axially displaced to alter the dimensions of the cavity and thereby alter the resonant frequency without any vertical displacement of rod 10 beyond the envelope. Since member 15 defines with end wall 2 a cavity which could produce spurious responses, a thin coating of an energy absorbent material such as poly-iron may be provided along the inner surface of wall 2. Member 15 may also be provided with a choke in the manner well known in the art to prevent any coupling of energy into the region between it and the end wall 2.

An understanding of the present invention will be enhanced after consideration of theory of operation which follows. At resonance the cavity 19 will effectively define a quarter wavelength stub with the wavelength in a rectangular waveguide section operating in the TE mode given by the equation:

My 2 l a) where a is the free space wavelength; a is the wide inside dimension of the waveguide and s, is the relative dielectric constant.

Since by definition:

where c is the velocity of light and f is the frequency, Equation 2 may be substituted in Equation 1 to result in:

From Equation 3 it will be seen that the wavelength in the waveguide does not vary linearly with frequency when e,. =l in the case of air. When wall member 15 is at the end of rod 10 adjacent to window the wavelength is the normal guide wavelength. However, as the wall member 15 is moved axially away from window 5 upon rotation of rod the amount of dielectric material exposed in cavity 19 increases thereby increasing the average dielectric contant 6,. As a result the guide wavelength is reduced in comparison to a structure without a dielectric rod. Hence the overall travel of the wall member required to tune over a frequency band is reduced. It will be evident that the effect of the dielectric material on the rate of change with respect to frequency is more noticeable at the low frequency end of the band with the wall member furthest removed from the window assembly. The advantages of the present invention will be more clearly understood after consideration of the graph in Fig. 3 plotting the position of member 15 with and without a dielectric rod against resonant frequency. It will be noted from curve 30 illustrating results with a dielectric rod that a more nearly linear tuning rate is achieved.

With the effect on the tuning rate noted by the increase in average dielectric constant within the resonant cavity several modifications or alternative embodiments may be envisaged. Referring to Fig. :4, a dielectric rod 40 is shown similar to rod 10 with a' conical shaped insert 41 of a material different from that used for the rod material. With this configuration any compensation desired may be achieved and the tuning rate may be controlled as desired. Other insert shapes such as straight or stepped could also be employed and the insert may be positioned from either end or form the entire core of the rod. In the embodiment shown in Fig. 4 a one-half inch rod of the material known commercially as Formica was substantially altered by an insert of either Coors AI-200 ceramic or sapphire. It is desirable to have the threaded portion of rod 10 formed from the material having a lower dielectric constant value with the higher value material disposed internally because of the easier machinability of the lower value materials. In some applications where tuning compensation is not as critical or a narrower bandwidth is desired, it may also be possible to form rod 10 with a central axial passageway extending therethrough.

A further variation of the invention which may be incorporated by reason of the fixed position of the dielectric rod upon rotation is the use of a detent mechanism. With such a mechanism accurate reproducibility may be realized to tune the tube to a plurality of predetermined resonant frequencies. The detent mechanism could be mounted on either one or both of the narrow sides of the waveguide envelope. Alternatively the mechanism could be mounted on end wall 2 in register with the end of the rod.

A still further modification could be made in the thread of dielectric rod 10. By providing a multiple thread such as a three or four turn configuration, a greater distance of travel of the wall member per turn of the rod will result. Consequently tuning over a relatively wide frequency band may be accomplished with a fewer number of revolutions of the dielectric rod.

There is thus disclosed a tuning structure readily adapt able to all types of microwave transmission devices wherein the average dielectric constant within the resonant cavity is increased to thereby shorten the guide wavelength. In addition, no external structure protrudes beyond the waveguide structure as the device is tuned and by the selection of various dielectric materials any desired tuning compensation may be achieved. The invention has been described as it pertains to one selected embodiment, however, many modifications or alterations will readily occur to those skilled in the art. The appended claims, therefore, should be accorded the latitude of interpretation to include such modifications or alterat ions commensurate with the spirit and scope of the invention as defined.

What is claimed is:

1. A broadband ATR microwave transmission control device comprising a metallic envelope having a wall enclosure at one end and a demountable encapsulated gasfilled resonant window assembly at the opposite end, a rod of a dielectric material supported centrally within said envelope by means secured to said wall enclosure, the inner end of said rod terminating in close proximity to said window assembly, a movable wall member disposed on said rod parallel to and spaced from the window assembly to define a resonant cavity, said rod member being rotated to thereby axially move said wall member without altering the relationship of said inner end to the window assembly.

2. A broadband ATR microwave transmission control device comprising a section of rectangular waveguide closed at one end and having a demountable encapsulated gas-filled resonant window assembly at the opposite end, a rod of a dielectric material supported centrally within said section by means secured to said closed end, said support means being adapted to permit rotation of said od Witho an e t ca d s lac me t t er o the ner n Q a tl t rm n t in 9.? p x m y to a d W ndow assembly, a m ab e a me b r d s o e n said rod parallel to and spaced from the window assembly with its peripheral edges contacting the inner surfaces of said Waveguide section, the spacing between said wall member and the window assembly being axially adjustable by rotation of said rod without altering the relationship of said inner end to the window assembly.

3. A broadband microwave transmission control device comprising a section of rectangular waveguide closed at one end and having a demountable encapsulated gasfilled resonantwindow assembly at the opposite end, a rod of a dielectric material supported by said closed end and extending centrally within said section with its inner end terminating in close proximity to said window assembly, a movable short-circuiting wall disposed on said rod parallel to and spaced from said window assembly to define a resonant cavity, the spacing between said shortcircuiting wall and the window assembly being adjustable by rotation of said dielectric rod, and increasing amount of said dielectric rod being exposed within the resonant cavity as said short-circuiting wall is moved toward said closed end.

4. A broadband microwave transmission control device according to claim 3 wherein said rod is a composite of dielectric materials having different dielectric constant values.

5. An ATR tube comprising a section of rectangular waveguide having a metallic wall enclosing one end and an encapsulated gas-filled resonant window assembly mounted at the opposite end, a rotatable rod of a dielectric material supported by means secured to said metallic end wall and extending centrally throughout the length of said section with its inner end terminating in close proximity to said window assembly, said rod defining a screw thread over a major portion of its length, a movable wall member threadably disposed on said rod parallel to and spaced from said Window assembly to define a resonant cavity, said cavity being tuned to resonance by axial adjustment of said wall member upon rotation of said rod without altering the relationship of said inner end to the window assembly.

6. An ATR tube comprising a section of rectangular waveguide having a metallic end wall and an encapsulated gas filled resonant window assembly mounted at the opposite end, a threaded dielectric rod supported centrally within said section by means secured to said end wall with its inner end terminating in close proximity to said window assembly, said support means being adapted to permit rotation of said rod without any vertical dis placement thereof, a movable wall member threadably mounted on said rod parallel to said window assembly and contacting the inner wall surfaces of said waveguide by means of a plurality of metallic fingers disposed along the peripheral edge, said wall member being axially adjustable by rotating of said dielectric rod to thereby expose an increasing amount of dielectric material in the area adjacent the window assembly upon movement toward said end wall.

7. An ATR tube according to claim 6 wherein said dielectric rod is a composite of dielectric materials having different dielectric constant values with the material having the higher value being inserted within the material having the lower value.

8. A tunable ATR tube comprising a rectangular waveguide envelope having an evacuated gas-filled resonant window assembly enclosing an end thereof, a rotatable threaded dielectric rod centrally positioned within said envelope with its inner end in close spaced relationship to said window assembly, a movable wall member disposed within said envelope in threaded engagement with said rod and parallel to said window assembly, said movable wall member being axially displaced to alter the resonant frequency of the tube over a selected range of microwave frequencies by rotation of said rod, the rate of displacement of said movable member necessary to vary the resonant frequency being reduced by the presence of an increasing portion of the overall length of the dielectric rod in the area adjacent the window assembly.

9. A tunable ATR tube comprising a metallic envelope defined by a section of rectangular waveguide having a closed end wall and an evacuated resonant window assembly containing an ionizable atmosphere at reduced pressure enclosing the opposite end thereof, a rotatable threaded rod of a dielectric material supported by said end wall and extending along the central axis of said envelope to a point in close proximity to said window assembly, said dielectric rod having a metallic wall member threadably disposed thereon parallel to said window assembly and in contiguous relationship with all the inner wall surfaces of said envelope, said wall member being axially movable by rotation of said rod to thereby alter its displacement from said window assembly with out altering the relationship of said rod to the window assembly.

10. A tunable ATR tube according to claim 9 wherein said rotatable rod is a composite of a plurality of dielectric materials having dissimilar dielectric constant values with the material of the higher value disposed as a insert within the material having the lower value.

References Cited in the file of this patent UNITED STATES PATENTS 2,408,355 Turner Sept. 24, 1946 2,449,794 Steele Sept. 21, 1948 2,629,068 Gottschalk et a1. Feb. 17, 1953 2,791,720 Lesch May 7, 1957 

